Seriously.... there are a lot of ridiculously awesome antagonists in anime, especially those who are the main character in the plot. So who is your favorite anime baddy and what would the anime be like if he/she were to be on the same side as the protagonist/main character(s)?
Lelouch vi Brittania from Code Geass would have to be my fave... The guy comes off as totally evil to everyone else, but he has every right to be and has you as the viewer angry with any one who opposes him (especially Suzaku at the beginning of the second season; he's so self-righteous ).

Actually, Lelouch is the protagonist. Leading protagonist does NOT always mean "hero" or "good guy/girl". He is an anti-hero. Anti-heroes are also protagonists. Some of you here are confusing villains with anti-heroes (even Vegeta after the Frieza saga).

All the Bleach antagonists are pretty awesome, but especially some of the espada, and particularly Ulquiorra: unreal power that realistically should have blown Ichigo away, and really everything about his storyline made me tear up.

Akram from Haruka: he sends shivers down my spine. I can't decide whether to root for him or run away from him!